Apparatus and method for separating a glass sheet from a moving ribbon of glass

ABSTRACT

Disclosed are methods for compensating the varying weight of a glass ribbon the glass ribbon is drawn from a molten glass forming material, and an apparatus therefore. The weight compensating apparatus is configured to apply a force to the glass ribbon that is inversely proportional to the weight of the glass ribbon such that as the glass ribbon weight increases, the force applied to the glass ribbon by the weight compensating device decreases.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 61/526,367 filed on Aug. 23, 2011,the content of which is hereby incorporated by reference.

BACKGROUND

1. Field

The present invention relates to a method of cutting or separating asheet of glass from a moving ribbon of glass, and in particular,ensuring a stable orientation of the ribbon during the separating cycleby compensating for a varying weight of the glass ribbon.

2. Technical Background

Down draw processes like the fusion-draw process produce a continuousribbon of glass that transitions from a viscous glass-forming materialto an elastic solid as the glass descends from a forming body. As thelength of the glass ribbon grows, a point is reached when a cuttingapparatus cuts (separates) a glass sheet from the ribbon, therebyshortening the ribbon by the length of the glass sheet. In addition toshortening the ribbon, the weight of the ribbon decreases by the weightof the separated glass sheet.

In a typical downdraw process used to produce glass sheets used in themanufacture of display panels, it is important to minimize contactbetween portions of the ribbon that will form a part of the subsequentdisplay panel, the so-called quality area of the ribbon. Consequently,contact is sparing, and performed typically only at the edges of theribbon, which are later removed. Therefore, the glass ribbon is onlymoderately constrained by the drawing apparatus. More particularly, thefree end of the ribbon, from which the glass sheet is removed, is freelyhanging from upstream supports when not engaged by the cuttingapparatus. When a glass sheet is cut from the free end of the ribbon,the sudden reduction in weight that occurs when the glass sheet isremoved can cause the newly formed free end to spring into a new shape.This sudden change in shape can be propagated upstream through theribbon and perturb the drawing process. For example, the pulling forceon the ribbon produced by upstream pulling rolls can be disrupted, andstresses can be induced into the glass ribbon as the glass ribbontransitions from a viscous liquid to a solid as it passes through theglass transition temperature range. A method and apparatus thatcompensates for the change in weight of the ribbon would minimize oreliminate this source of process inconsistency.

SUMMARY OF THE INVENTION

As a continuously moving glass ribbon, e.g. a glass ribbon formed by thecontinuous down draw of molten glass from a reservoir, the shape of theglass ribbon varies with time. That is, as the glass ribbon growslonger, the weight of the glass ribbon increases. Consequently, theglass ribbon tends to flatten longitudinally over time, until a glasssheet is removed from the glass ribbon. Since a portion of the glassribbon hangs freely, when the glass sheet is removed from the glassribbon, the weight of the glass sheet is also removed, and the forcesthat tended to longitudinally flatten the glass ribbon are suddenlyreduced or eliminated. The resultant movement of the glass ribbon mayinduce perturbations into the glass ribbon that disrupt portions of theglass ribbon upstream of the where the glass sheet from removed from theribbon. For example, these perturbations may negatively influence theglass ribbon in that portion of the glass ribbon where the ribbontransforms from a viscous liquid to an elastic solid. One potentialeffect can be the freezing in of stresses. Other effects includedisruptions to the drawing forces applied to the glass ribbon.

In accordance with embodiments described herein below, apparatus andmethods are described wherein the foregoing variation in ribbon weightover time is compensated for by a weight compensating apparatus.

In one embodiment a method of forming a glass sheet is disclosedcomprising flowing molten glass from a forming body in a downdrawprocess to form a continuously moving glass ribbon having a length and afree end, and wherein the length of the glass ribbon varies as afunction of time, engaging the continuously moving glass ribbon with afirst weight compensating apparatus, applying a downward force to thecontinuously moving glass ribbon with the first weight compensatingapparatus, wherein the force applied by the first weight compensatingapparatus is inversely proportional in magnitude to the length of theglass ribbon such that as the length of the glass ribbon increases, theforce applied by the weight compensating device decreases. A score maythen be formed in the continuously moving glass ribbon and a portion ofthe continuously moving glass ribbon separated at the score to form aglass sheet. The preceding steps may then be repeated to form asubsequent glass sheet, wherein the weight compensating apparatuscontinues in a reciprocating fashion to engage the glass ribbon andapply a downward force to the glass ribbon, disengage from the glassribbon, move upward to a home position and re-engage the glass ribbon tobegin another cycle.

The embodiment may further comprise engaging the continuously movingglass ribbon with a traveling anvil machine, wherein the traveling anvilmachine comprises a scoring device and a backing bar. Preferably, thetraveling anvil machine does not apply a downward force to thecontinuously moving glass ribbon. Additionally, the method may alsoinclude engaging the glass ribbon with a robot below the score, whereinthe robot bends the glass ribbon across the score to produce a crackthat separate a glass sheet from the continuously moving glass ribbon.

Preferably, the first weight compensating apparatus engages with a firstedge portion of the continuously moving glass ribbon, thereby avoidingthe interior quality area of the glass ribbon. The quality area is thearea of the glass ribbon that continues through the glass making processand is useable in subsequent final products such as display panels,lighting panels and the like. The edge portions of the glass ribbon aretypically cut away and discarded, or used as cullet in the glass makingprocess.

In certain embodiments the continuously moving glass ribbon is engagedwith a second weight compensating apparatus. The second weightcompensating apparatus preferably engages the glass ribbon on a secondedge portion. In some embodiments the second weight compensatingapparatus is controlled independently from the first weight compensatingapparatus such that a downward force applied to the glass ribbon by thesecond weight compensating apparatus is different than the force appliedby the first weight compensating apparatus.

In another embodiment, a method of forming a glass sheet is describedcomprising flowing molten glass from a forming body in a downdrawprocess to form a continuously moving glass ribbon having a free end,the glass ribbon traveling with a velocity vector V and comprising atime varying free weight, engaging the glass ribbon with a first weightcompensating apparatus, wherein the first weight compensating apparatusapplies a first force to the glass ribbon that is inversely proportionalin magnitude to the free weight; scoring the glass ribbon; andseparating a glass sheet from the glass ribbon. The separating step maycomprise, for example, applying a bending force to the glass ribbon witha robot engaged with the glass ribbon. In some embodiments, the methodmay further comprise engaging the glass ribbon with a backing bar, thebacking bar traveling at a velocity vector that equal the velocityvector V of the glass ribbon.

Preferably, first weight compensating apparatus comprises an engagingdevice and the step of engaging comprises extending an actuator towardthe glass ribbon from the engaging device and contacting the glassribbon with a suction cup coupled to the actuator. The force applied bythe weight compensating device may in some instances be greater than thefree weight at a given time.

The method may further comprise engaging the glass ribbon with a secondweight compensating apparatus. The second weight compensating apparatusneed not apply a force that is the same magnitude as the first weightcompensating device, but may apply a force to the glass ribbon that isdifferent that the force applied by the first weight compensatingapparatus.

In still another embodiment, an apparatus for producing a glass sheet isdisclosed comprising a forming body for forming a glass ribbon frommolten glass; a weight compensating apparatus for applying a force tothe glass ribbon, the weight compensating apparatus comprising; a lineardrive unit; a drive motor coupled to the linear drive unit; an engagingdevice coupled to the linear drive unit, the engaging device comprisinga suction cup for contacting the glass ribbon; and wherein the weightcompensating apparatus is configured to apply a downward force to theglass ribbon that is inversely proportional to a length of the glassribbon. The engaging device may comprise an actuator configured to movethe suction cup toward or away from the glass ribbon. In someembodiments the apparatus may comprise a plurality of weightcompensating apparatus. The forming body preferably comprises a channelformed in an upper surface of the forming body for receiving moltenglass, and converging forming surface that join at a root. Molten glassoverflowing the channel walls flows over the converging forming surfacesas separate flows that rejoin or fuse at the root of the forming body.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and constitute a part of this specification. The drawingsillustrate various embodiments of the invention and, together with thedescription, serve to explain the principles and operations of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of an exemplary glass making system;

FIG. 2 is an elevational view of the glass making system of FIG. 1showing components of the system downstream of the pulling rolls;

FIG. 3 is a front view of a traveling anvil machine and a weightcompensating apparatus according to an embodiment of the presentinvention;

FIG. 4 shows an edge view of various longitudinal shapes of acontinuously moving glass ribbon as the glass ribbon length increases;

FIG. 5 is a front view of the apparatus of FIG. 2 showing components forcontrolling movement of and force applied by the weight compensatingapparatus of FIG. 3.

FIG. 6A-6F illustrate stages of an exemplary glass ribbon cutting cycleas viewed from an edge of the glass ribbon.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, example embodiments disclosing specific details are setforth to provide a thorough understanding of the present invention.However, it will be apparent to one having ordinary skill in the art,having had the benefit of the present disclosure, that the presentinvention may be practiced in other embodiments that depart from thespecific details disclosed herein. Moreover, descriptions of well-knowndevices, methods and materials may be omitted so as not to obscure thedescription of the present invention. Finally, wherever applicable, likereference numerals refer to like elements.

FIG. 1 illustrates an exemplary embodiment of a fusion glass makingsystem 10 for forming a glass sheet comprising melting furnace 12,fining vessel 14, stirring vessel 16, receiving vessel 18, downcomer 20,inlet 22 and forming body 24 from which a thin, continuously movingribbon 26 of a molten glass-forming material descends. Glass makingsystem 10 further comprises various other vessels or conduits forconveying the molten glass-forming material, including amelter-to-fining vessel connecting tube 28, a fining vessel-to-stirringvessel connecting tube 30, and a stirring vessel-to-receiving vesselconnecting tube 32. While the melting furnace and forming body aretypically formed from a ceramic material, such as ceramic brickscomprising alumina or zirconia, the various vessels and pipingtherebetween often comprise platinum or an alloy thereof. Although thefollowing description relates to an exemplary fusion downdraw system andprocess, such as the system illustrated in FIG. 1, the present inventionis equally applicable to other variations of downdraw glass makingprocesses such as a single sided overflow process or a slot drawprocess, which processes are well known to those skilled in the art.

In accordance with the exemplary fusion process of FIG. 1, meltingfurnace 12 is provided with a batch material 36 that, as denoted byarrow 38, is charged into furnace 12 and melted by the furnace toproduce a glass-forming material (hereinafter molten glass 40). Moltenglass 40 is conveyed from melting furnace 12 to fining vessel 14 throughmelting furnace-to-fining vessel connecting tube 28. The molten glass isheated to a temperature in excess of the furnace temperature in finingvessel 14, whereupon multivalent oxide materials contained within themolten glass release oxygen that rises through the molten glass. Thishigh-temperature release of oxygen aids in removing the small bubbles ofgas within the molten glass generated by the melting of the batchmaterial.

The molten glass then flows from fining vessel 14 through finingvessel-to-stirring vessel connecting tube 30 into the stirring vessel 16where a rotating stirrer mixes and homogenizes the molten glass toensure an even consistency. The homogenized molten glass from stirringvessel 16 then flows through stirring vessel-to-receiving vesselconnecting tube 32, is collected in receiving vessel 18 and routed toforming body 24 through downcomer 20 and inlet 22 where the molten glassis formed into a glass ribbon.

Forming body 24 comprises an open channel 42 positioned on an uppersurface of the forming body and a pair of converging forming surfaces44, best seen in FIG. 2, that converge at a bottom or root 46 of theforming body. The molten glass supplied to the forming body flows intothe open channel and overflows the walls thereof, separating into twoindividual flows of molten glass that flow over the converging formingsurfaces. When the separate flows of molten glass reach the root, theyrecombine, or fuse, to form a single glass ribbon 26 of viscous moltenglass that descends from the root of the forming body. Glass ribbon 26cools as it descends and passes through a glass transition temperaturerange where the glass ribbon undergoes a transformation from a viscousliquid to an elastic solid. Pulling rolls 48 arranged in opposingcounter-rotating pairs contact and pinch the viscous glass ribbon alongthe edges of the ribbon and aid in drawing the ribbon in a downwarddirection. While pulling rolls 48 are driven, such as by suitablemotors, additional driven or non-driven rolls may also contact the edgesof the ribbon to aid both in guiding the ribbon and maintaining a widthof the ribbon against naturally occurring surface tension effects thatwork to otherwise reduce the width of the ribbon.

To separate a glass sheet from the ribbon, a traveling anvil machine(TAM) 50 shown in FIG. 2 is employed. In a typical arrangement and asbest viewed with the aid of FIG. 3, TAM 50 includes an anvil or backingbar 52 and a scoring device 54. Scoring device 54 may comprise, forexample, a scoring wheel 56 such as that depicted in FIG. 3 thatcontacts glass ribbon 26 and applies a force against first major surface58 of the glass ribbon across a width thereof, or a portion of thewidth, to produce a score line 60. As the name implies, anvil or backingbar 52 provides a counter force to the second major surface 62 of theribbon (the side of the ribbon opposite the side on which the score lineis made), thereby minimizing movement of the ribbon as the scoring wheelmoves width-wise or laterally across the glass ribbon. Thus, inoperation, the backing bar contacts the glass ribbon on a surface orside of the ribbon opposite the side of the ribbon on which the score isformed. During such time that a cutting operation is not occurring,scoring device 54 and backing bar 52 may be retracted away from theglass ribbon.

Because glass ribbon 26 is moving during the scoring process, TAM 50moves with the glass ribbon to facilitate formation of score line 60that is perpendicular to the edges 64 of the glass ribbon. That is, theribbon is moving with a certain velocity vector V comprising a scalarmagnitude (speed) dictated in part by the rotational speed of thepulling rolls, and a direction. It will be assumed for the purpose offurther discussion that the direction of V is vertically downward,although in some embodiments, the direction may be different, such as byoffsetting one or more sets of rolls.

To ensure no relative movement of TAM 50 and glass ribbon 26 during thescoring process, TAM 50 is configured to move with a speed and directionrepresented by velocity vector S that is the same or substantially thesame as velocity vector V of the ribbon. Therefore, as scoring device 54moves across the width of the ribbon, there is substantially no velocitydifference between the downward movement of the ribbon and the downwardmovement of the TAM (and therefore the scoring wheel, or other scoringcomponent) in the direction of travel of the glass ribbon. To wit,scoring device 54 is able to produce a score line transverse to thedirection of ribbon travel.

When the score line is completed, robot 66 engages the portion of theribbon downstream of the score line relative to the direction of travelof the ribbon with a plurality of suction cups 68, and applies a bendingmoment to the ribbon in a direction perpendicular to a major surface ofthe ribbon. Preferably, robot 66 contacts the glass ribbon on the samemajor surface that was contacted by backing bar 52, i.e. second majorsurface 62. The bending moment induced by robot 66 produces a tensilestress across score line 60 that causes a crack to form at the scoreline. The crack propagates through the thickness of the glass ribbon,thereby separating a glass sheet from the ribbon. Backing bar 52, andany additional contacting nosing bars that may be employed, are thendisengaged from the glass ribbon and TAM 50 moves upstream to itsstarting position in preparation for another cutting cycle. In additionto the backing bar, TAM 50 may include one or more stabilizing bars(nosing bars) that contact the glass ribbon upstream, or downstream ofthe backing bar, either on the same side of the glass sheet as thebacking bar (first major surface 58), or on the side of the glass sheetopposite the backing bar (second major surface 62) to further isolatemovement below the backing bar or nosing bar from propagating upward tothe viscous elastic zone of the glass ribbon. For the purpose of furtherdiscussion, the range of motion of the TAM during the cutting cycle inthe direction of ribbon travel will be referred to as the TAM stroke.Accordingly, as arbitrarily chosen here, the cutting cycle begins justafter a sheet of glass has been removed from glass ribbon 26 and TAM 50has moved to the top of its stroke. It should be noted that the TAMstroke need not equate to the length of a glass sheet to be separatedfrom the ribbon, but can be shorter than a glass sheet length.

At the beginning of a cutting cycle, the ribbon is at a minimum, ornear-minimum length relative to root 46 of forming body 24, the removalof a glass sheet 70 (see FIGS. 6D and 6E) of a predetermined lengthhaving just occurred. Additionally, the weight of the glass ribbonrelative to a given reference point, is also at a minimum. For example,if pulling rolls 48 are the lowest contact point between free end 72 ofglass ribbon 26 and that contact point, the weight of the glass ribbonover that length of ribbon is at a minimum. There may be additionalrolls positioned lower than the pulling rolls that may support a weightof the ribbon, so the use of the pulling rolls as a reference point isarbitrary and used herein for the purpose of illustration, notlimitation. Indeed, a length of the ribbon may be in respect of anyarbitrary reference point, such as the root of the forming body. It issufficient to say that at some point along the length of glass ribbon 26between and including root 46 of forming body 24 and free end 72 of thecontinuously moving glass ribbon there is a structure that contacts theglass ribbon, and may support the weight, or a portion of the weight, ofthe ribbon, and the distance between that structure and the free end ofthe glass ribbon represents a weight based on the density of the glassforming the glass ribbon and the dimensions of the glass ribbon. Becausethe glass ribbon is continuously being renewed from the forming body,and is therefore continuously moving downward, the length of the glassribbon below the selected reference point and terminating at the freeend of the glass ribbon, hereafter the “free length” (FL) of the ribbon,is continuously increasing during intervals of cutting. And with anincreasing free length there is a continuously increasing weightassociated with the free length, hereinafter the “free weight” (FW).That is, the free length and the free weight at the beginning of acutting cycle are at a minimum, and increase substantially steadilyuntil a glass sheet is separated from the glass ribbon, at which pointthe free length and free weight return to the minimum for each. At thepoint just prior to the separation of the glass sheet from thecontinuously moving glass ribbon, the free length and the free weightare at a maximum (for the purpose of discussion, any load, or weightassumed by the robot in contact with the free length of the ribbon isnot considered). The free length is at a minimum at the moment a glasssheet is separated from the ribbon, and at a maximum the moment before aglass sheet is separated from the ribbon. Thus, the free length FL canbe represented by L_(min)+ΔL, where L_(min) is a minimum length of thefree length and ΔL represents the variation in the free length, which isalso the length of the glass sheet to be separated from the glassribbon. Similarly, the free weight FW is represented by W_(min)+ΔW whereW_(min) is a minimum weight of the free length and ΔW represents thevariation in the free weight, which is also the weight of the glasssheet to be separated from the glass ribbon. Accordingly, ΔW_(max) andΔL_(max) denote the maximum change in free weight ΔW and the maximumchange in free length ΔL, respectively.

As the free length and free weight increase, the shape of the glassribbon is constantly changing. For example, the glass ribbon may exhibita bow or curl. As the free weight increases, there is a tendency for theribbon to flatten, at least longitudinally along a length of the glassribbon, as a result of increase in free weight. That is, as the ribbondescends from the forming body, it may exhibit many complex shapes,either along a length of the ribbon, across a width of the ribbon, or acombination thereof such that the ribbon has a non-planar overall shape.The steadily increasing free weight has a tendency to reduce thenon-planar shape of the ribbon along at least a length of the ribbon.However, the free length of the ribbon, being in an elastic portion ofthe ribbon, holds a particular longitudinal shape as a consequence ofthe free weight. When the free weight is suddenly removed, the glassribbon will assume the shape it would have had without the free length.This can be more easily seen with the aid of FIG. 4, which illustrates alongitudinal shape of the glass ribbon as seen from an edge of theribbon during various stages of a single cutting cycle. In accordancewith FIG. 4, curve 74 represents a shape of the glass ribbon at thestart of a cutting cycle: the free length FL and free weight FW are at aminimum (ΔL and ΔW are equal to zero). The dashed line represents theposition of score line 60. As the free length FL and the free weight FWincrease, the ribbon begins to flatten, as depicted by curves 78 and 80,respectively. Finally, just before the glass sheet is separated at scoreline 60, the free length and free weight are at a maximum represented bycurve 82 and the glass ribbon is at its flattest. However, because atleast a portion of the free length is within an elastic region of theribbon, there is significant energy stored in the free length when thefree length is at a maximum and weighted down by the free weight. Whenglass sheet 70 is removed (that portion of the ribbon between score line60 and free end 72 of the glass ribbon), the potential energy in theglass ribbon is released and the ribbon springs back to the position ofcurve 74. As noted, this sudden shape change can cause upstreamdisruption to the drawing process.

Accordingly, FIG. 3 further illustrates a weight compensating apparatus84 that compensates for the varying free weight, thereby stabilizing ashape of the continuously moving glass ribbon. Weight compensatingapparatus 84 comprises an engaging device 86 that contacts and engageswith glass ribbon 26, and a conveyor 88 along which engaging device 86travels with a velocity vector U that is the same or substantially thesame as the velocity vector V of the continuously moving glass ribbon.It is noted that velocity vector U is shown in the downward directiononly. When the direction of engaging device 86 is reversed so thatengaging device is moving upward, the speed at which the upward movementoccurs can be considerably faster than the scalar speed comprisingvelocity vector U. Also, as noted herein below, engaging device 86 maybe moved at a speed somewhat different than the speed component ofvelocity vector V as a way of developing a downward force on ribbon 26when engaged with the ribbon. This velocity differential will vary asthe free weight of the ribbon varies. While the following description isdirected to a single weight compensating device, multiple weightcompensating devices may be used. For example, two weight compensatingdevices may be used to apply a load to each side edge portion of theglass ribbon.

Engaging device 86 may include, for example, a suction cup 90 formedfrom a resilient material much softer than the glass of the glass ribbonso that minimal damage may be incurred from contact between the suctioncup and the glass ribbon. Suction cup 90 may be coupled to actuator 92configured to extend or retract the suction cup toward or away from amajor surface of the glass ribbon, respectively. Actuator 92 maycomprise, for example, a pneumatic cylinder in fluid communication witha source of pressurized air (not shown). Alternatively, actuator 92 maycomprise a hydraulic cylinder or an electrical solenoid.

Suction cup 90 is connected with a vacuum source (not shown) so that avacuum is supplied to the suction cup and the suction cup grips theglass ribbon when the suction cup is brought into contact with the glassribbon. Preferably, suction cup 90 contacts the glass ribbon at theedges of the ribbon, outside the quality area, since the edges of theglass ribbon are later removed.

Engaging device 86 preferably also includes a load cell (not shown) thatmeasures a torque T_(q) between the glass ribbon and the engagingdevice.

Engaging device 86 is coupled to conveyor 88. Conveyor 88 comprises arail or linear drive unit 94 and drive motor 96. Drive motor 96 may be,for example, a servomotor. Drive motor 96 is coupled to linear driveunit 94 and together they provide motion and torque control to engagingdevice 86 as instructed by control unit 98 (See FIG. 5). Control unit 98may be a computing device, such as a general purpose computer or otherinformation processing device, that receives process inputs from thedrawing equipment and sends out a control signal that varies a drivespeed of linear drive unit 94 via drive motor 96.

As illustrated in the diagram of FIG. 5, control unit 98 may beconfigured to receive a rotational speed of the pulling rolls (oralternatively a rotational speed of one or more other rolls in contactwith the continuously moving glass ribbon) obtained from, for example,an encoder incorporated into the pulling rolls and provided to controlunit 98 through control line 100. Control unit 98 can then calculate alinear speed of the continuously moving ribbon from the rotation speedof the pulling rolls. However, other methods of obtaining a linear speedof the glass ribbon as are known in the art may be employed.

Control unit 98 may be configured to determine a rate of free weightincrease based on the linear speed of the glass ribbon. This can bedetermined, for example, based on the glass density, which is known fora known glass composition, and the dimensions of the glass ribbon widthand free length. Thus, control unit 98 can calculate a rate of change ofthe free weight and accordingly calculate the drive torque to be appliedto linear drive unit 94 and therefore the downward force F applied byweight compensating apparatus on the glass ribbon so as to emulate themaximum free weight. That is, control unit 98 may be configured viaprocess inputs and internal software to provide a signal to weightcompensating apparatus 84 through control line 102 to apply a downwardforce on the glass ribbon that emulates the force derived from themaximum free weight. This downward force F can be applied, for example,by moving engaging device 86, coupled to conveyor 88, at a speedsomewhat greater than the speed component of ribbon velocity V. When thefree weight is at a minimum, weight compensating apparatus 84 applies adownward force on the glass ribbon based on a predetermined length for aglass sheet so that the ribbon behaves (i.e. assumes a shape) as itwould if the maximum free length (and therefore free weight) waspresent. As the free length of the continuously moving glass ribbonincreases, the downward force applied to the glass ribbon by weightcompensating apparatus 84 decreases proportionally. When the free weightis at a maximum, the force applied by the weight compensating apparatusis at a minimum. Accordingly, the combination of the instantaneous freeweight, and the instantaneous value of force F combine so that the shapeof the glass ribbon remains substantially constant through the cuttingcycle.

In some embodiments, pulling rolls (or other rolls in contact with theglass ribbon) may be configured to supply a torque value to control unit98. For example, the torque T_(q) developed at the pulling rolls can beused to determine an actual present (instantaneous) weight of the ribbonrather than a weight based on a calculated length (itself based on aspeed of the ribbon). Moreover, control unit 98 may be used to controlthe operation of TAM 50 through control line 104 and robot 66 throughcontrol line 106.

Referring to FIGS. 6A-6F, operation of weight compensating apparatus 84will now be described through a full cutting cycle beginning arbitrarilyat a point where a glass sheet has just been removed from thecontinuously moving glass ribbon and the TAM and weight compensatingapparatuses are at their respective initial positions. For the purposeof describing the operation of the TAM and the weight compensatingapparatus, the position of TAM 50 at the beginning of a cut cycle willbe referred to as start position 108, which is the upper-most upstreamposition of TAM 50 at the top of the TAM stroke; the position ofengaging device 86 will be referred to as home position 110, which isthe upper-most upstream position of the engaging device. In respect oftiming, the time at the start of the cut cycle will be arbitrarilydenoted as t₁.

For the purpose of discussion, return travel times of TAM 50 andengaging device 86 will be considered negligible. In practice, allowancewould be made for finite return travel times. That is, for example,while the TAM takes a finite amount of time to travel over the device'sentire stroke during a return motion, for the purposes of discussion,this return travel time, and inertia (affecting time to come to fullspeed) will be considered negligible.

Beginning at t₁ and referring to FIGS. 5 and 6A, engaging device 86 ofweight compensating apparatus 84 is engaged with glass ribbon 26, and istraveling downward with velocity vector U substantially equal tovelocity vector V of glass ribbon 26. To wit, actuator 92 is activated,extending suction cup 90 toward glass ribbon 26. A vacuum is applied tosuction cup 90. When suction cup 90 contacts glass ribbon 26, suctioncup 90 engages with glass ribbon 26 so that a downward force F can beapplied to the glass ribbon without suction cup 90 losing contact orchanging position (slipping). Drive motor 96, controlled by control unit98, moves engaging device 86 downward via linear drive unit 94.Simultaneously, a load cell incorporated into engaging device 86 sensesthe torque T_(q) experienced by the engaging device and provides themagnitude of the torque to control unit 98 through control line 102. Thetorque T_(q) is converted to an equivalent downward force F by controlunit 98, which thereafter controls drive motor 96 so that downward forceF applied by the engaging device equals a predetermined value, e.g. ΔW.Preferably, downward force F at t₁ where the free length is at a minimumis the same, or nearly the same as ΔW_(max). In other words, the purposeof weight compensating apparatus 84 being to compensate for variationsin the free weight (i.e. ΔW), the weight compensator applies a maximumforce when ΔL=0 and a minimum force at ΔL_(max). In a preferredembodiment ΔW_(max) is the weight of the glass sheet to be removed fromthe glass ribbon. That is, since the length of the glass ribbon ispredetermined by the process (e.g. a customer specification), the weightof that glass sheet is predetermined and known or at leastwell-estimated. However, ΔW_(max) can be more than or in some cases lessthan the predetermined weight of the glass sheet to be removed. This mayoccur, for example, when the process is changed to produce a glass sheethaving a different length than a previous glass sheet, but the loadapplied by the weight compensating apparatus is not changed. As thecontinuously moving glass ribbon moves downward, the free length, andtherefore ΔW, increases and control unit 98 controls drive motor 96 sothat the load applied by engaging device 86 is proportionally decreased.Additionally, while TAM 50 is illustrated as being arranged opposite asingle side of glass ribbon 26, in practice, TAM 50, or portionsthereof, may be arranged on both sides of the glass ribbon. This may beself evident, since the scoring takes place on one side of the glassribbon, while the backing bar is arranged opposite the scoring device.

Beginning at t₂ as shown in FIG. 6B and referencing FIG. 5 also, theglass ribbon free length continues to grow by ΔL and score line position114 nears the position of TAM 50. When a sufficient length of glassribbon has passed TAM 50 that TAM 50 is adjacent the intended score lineposition 114, TAM 50 engages with glass ribbon 26. Position 116represents a subsequent (future) scoring position to produce the nextglass sheet.

Beginning at time t₃ as depicted in FIG. 6C and referring also to FIG.5, when free end 72 has reached a predetermined position where the freelength, and therefore the free weight has reached a maximum(ΔL=ΔL_(max), ΔW=ΔW_(max)), and the load applied by weight compensatingapparatus 84 has reached a minimum (F is minimized), engaging device 86is disengaged from glass ribbon 26 and returned to home position 110.Robot 66 engages with glass ribbon 26 and scoring device 54 beginsscoring across a width of the glass ribbon, producing score line 60.Robot 66 engages with glass ribbon 26 below score line 60.

As shown by FIG. 6D and beginning at time t₄, robot 66 applies a bendingforce across score line 60. The bending force is manifest as a tensilestress that creates a crack at the score line that propagates throughand across the glass ribbon.

Referring now to FIG. 6E and beginning at time t₅, engaging device 86 isengaged with glass ribbon 26 and traveling with velocity vector U thatis the same or substantially the same as velocity vector V of the glassribbon 26. Meanwhile, robot 66, removes the separated glass sheet 70.

Finally, at t₆ and as shown in FIG. 6F, TAM 50 disengages from glassribbon 26 and moves back to start position 108 and engaging device 86 issupplying a maximum downward force F.

One skilled in the art will understand based on the present disclosurethat the foregoing sequence and timing of events may be modified asneeded for a particular glass drawing equipment and operation, and thatthe preceding sequence was merely a description of an exemplarysequence.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of forming a glass sheet comprising: a)flowing molten glass from a forming body in a downdraw process to form acontinuously moving glass ribbon having a free length and a free end,and wherein the free length of the glass ribbon is defined as the lengthof the glass ribbon from the lowest contact point between the glassribbon and a structure supporting a weight of the glass ribbon to thefree end of the glass ribbon, and where the free length varies as afunction of time; b) engaging the continuously moving glass ribbon alongthe free length with a first weight compensating apparatus, the firstweight compensating apparatus comprising an engaging device thatcontacts and engages the continuously moving glass ribbon and aconveyor, the engaging device traveling along the conveyor in a downdrawdirection of the continuously moving glass ribbon; c) applying a firstdownward force to the continuously moving glass ribbon with the firstweight compensating apparatus, wherein the first downward force appliedby the first weight compensating apparatus is inversely proportional inmagnitude to the free length of the glass ribbon such that as the freelength of the continuously moving glass ribbon increases, the forceapplied by the weight compensating apparatus decreases; d) forming ascore in the continuously moving glass ribbon; and e) separating aportion of the continuously moving glass ribbon at the score to form aglass sheet.
 2. The method according to claim 1, further comprisingrepeating steps b) through d) to form a subsequent glass sheet.
 3. Themethod according to claim 1, further comprising engaging thecontinuously moving glass ribbon with a traveling anvil machine.
 4. Themethod according to claim 3, wherein the traveling anvil machine doesnot apply a downward force to the continuously moving glass ribbon. 5.The method according to claim 1, further comprising engaging thecontinuously moving glass ribbon with a robot below the score.
 6. Themethod according to claim 1, wherein the first weight compensatingapparatus is engaged with a first edge portion of the free length. 7.The method according to claim 1, further comprising engaging a secondedge portion of the free length with a second weight compensatingapparatus.
 8. The method according to claim 7, wherein the second weightcompensating apparatus is controlled independently from the first weightcompensating apparatus.
 9. The method according to claim 8, wherein thefirst force applied to the continuously moving glass ribbon by the firstweight compensating apparatus varies from a second force applied to thecontinuously moving glass ribbon by the second weight compensatingapparatus.
 10. The method according to claim 1, wherein the structuresupporting the free length of the glass ribbon comprises rollers.
 11. Amethod of forming a glass sheet comprising a) flowing molten glass froma forming body in a downdraw process to form a continuously moving glassribbon having a free end, the glass ribbon traveling with a velocityvector V and comprising a time varying free weight; b) engaging theglass ribbon with rollers; c) engaging the glass ribbon below a lowestroller engaged with the glass ribbon with a first weight compensatingapparatus, the first weight compensating apparatus comprising anengaging device that contacts and engages the glass ribbon and aconveyor, the engaging device traveling along the conveyor in a downdrawdirection of the glass ribbon, wherein the first weight compensatingapparatus applies a first force to the glass ribbon that is inverselyproportional in magnitude to the variation in free weight; d) scoringthe glass ribbon; and e) separating a glass sheet from the glass ribbon.12. The method according to claim 11, wherein step e) comprises applyinga bending force to the glass ribbon with a robot engaged with the glassribbon.
 13. The method according to claim 11, further comprisingengaging the glass ribbon with a backing bar, the backing bar travelingat a velocity vector that equals the velocity vector V of the glassribbon.
 14. The method according to claim 11, wherein the first weightcompensating apparatus comprises an engaging device and the engagingbelow the lowest roller comprises extending an actuator toward the glassribbon from the engaging device and contacting the glass ribbon with asuction cup coupled to the actuator.
 15. The method according to claim11, wherein the force applied by the weight compensating device isgreater than the free weight at a predetermined time.
 16. The methodaccording to claim 11, further comprising engaging the glass ribbonbelow the lowest rollers engaged with the glass ribbon with a secondweight compensating apparatus.
 17. The method according to claim 16,wherein the second weight compensating apparatus applies a force to theglass ribbon that is different than the force applied by the firstweight compensating apparatus.
 18. A method of forming a glass sheetcomprising: a) flowing molten glass from a forming body in a downdrawprocess to form a continuously moving glass ribbon; b) engaging theglass ribbon with rollers, the glass ribbon comprising a free enddisposed below the rollers and a free length disposed between therollers and the free end, and wherein a weight of the free length variesas a function of time; b) engaging the free length with a first weightcompensating apparatus, the first weight compensating apparatuscomprising an engaging device that contacts and engages the glass ribbonand a conveyor, the engaging device traveling along the conveyor in adowndraw direction of the glass ribbon; c) applying a first downwardforce to the free length with the first weight compensating apparatus,wherein the first downward force applied by the first weightcompensating apparatus is inversely proportional in magnitude to thefree weight of the glass ribbon such that as the free weight of thecontinuously moving glass ribbon increases, the force applied by theweight compensating apparatus decreases; c) forming a score in thecontinuously moving glass ribbon; and d) separating a portion of thecontinuously moving glass ribbon at the score to form a glass sheet. 19.The method according to claim 18, wherein engaging the free lengthcomprises extending an actuator from the first weight compensatingdevice toward the glass ribbon and contacting the free length with asuction cup coupled to the actuator.
 20. The method according to claim18, further comprising engaging the free length with a second weightcompensating apparatus.